Aerofoil Type, need a picture

[Kyle G.]

Alright guys, I need a little assistance. What type of aerofoil would work for a 40 inch span 5 inch chord and fully semetrical. I'm just looking for a proven aerofoil. It's got to be 1/2 inch thick too. I just need a picture, I could resize it. But I don't know what works and doesn't. Thanks :)

[davidfee]

Take a sheet of paper and draw a line 5" in length.
Measure 1.25" from one end and mark this distance.
Measure and mark 0.25" above and below the line at this mark.
Use a french curve, your shoe, etc. to connect the dots.

Are you looking for minimum drag at a given Cl, or what? Really anything will work at these low Reynolds numbers.

-David

[Kyle G.]

Thanks David, Generally low drag and higher lift (how possible is that with a fully semetrical wing?) Wind penetration would be nice. I'm totally new with making my own wings and the whole principle of reynolds numbers and all that jazz, so any knowledge would be of great assistence. Tomorrow I'll give your measurments a try on paper and see how good my "french curve" is and hopefully I'll have an aerofoil that works? This is goin on a 40 inch span, Cessna Caravan floatplane in the 15-25 oz range to give an idea of what this wing will be lifting. (kinda like the one in my avator :) )

[Mike Taylor]

Here's an aerobatic airfoil designed for R/C models. It performs well at moderate Rn.

[Ollie]

See:
http://www.nasg.com/afdb/show-airfoil-e.phtml?id=1085

[BMatthews]

At your numbers I would suggest you forget the thickness limit and use a Selig 8020. It's designed for a higher angle of attack stall and low drag at low angles. It may come out a bit thicker or thinner than your spec of 1/2 inch but it'll be close. Let it be thicker if need be.

[Kyle G.]

BMatthews, how thick would it be though? The reason I want it to be 1/2 inch is because I've already got my wing spare together and it will be hard to make it thicker then that.

[nemonis]

Kyle:

Looking at data for the Selig-Donovan SD8020 airfoil, it should fit your criteria quite well. It is 10.1% thick, which translates into 0.505 inches at the thickest point. Based on the coordinates, the thickest point is 28.3% aft of the leading edge, which in your case would be 1.415 inches from the leading edge.

Looking at xfoil, it looks like this airfoil would work well for your plane. It looks like stall speed would be around 20-25 mph, based on your weight range. I am still really new to this design analysis thing, so it anyone feels the need to correct me, please do so.

God bless!
Wayne J.

[Sparky Paul]

SD 8020 looks like this...

[Kyle G.]

Cool thanks Wayne and Sparky, I started cutting out that aerofoil and it looks nice so far. Now heres my question. The wing tapers off to about 3.5 inches at the tip. Do I want to decrease the size of the whole rib/aerofoil (IE resize the image to 3.5 inches long and cut) or do I want to keep the same thickness and just sort of cut it shorter. Or a combo of the two? I feel that that might be critical getting it to fly right, so I'll await an answer until I finish cutting them out. Thanks! :)

Kyle

[Sparky Paul]

For a plane that small, there is little that is "critical" on the wing.
Smooth curves with no major bumps in the surface count for more than the shape of the rib.

[Highflight]

I would only add that I understand that you want to keep it thin to meet mechanical fit to the fuse, but the ratio you want is more in line with what would be used for tail feathers, not a wing.
Plus, you want to put floats on the plane which means you're going to have to significantly overpower the plane to get it off the water because that wing sure won't want to help much.

If you go with your plan, that's going to be one zippy little model and will probably fly more like a scale Learjet than a Cessna Caravan.

I don't even want to suggest any particular wing because nothing I could offer would fit your .500" thickness requirement.
In short, you can't get there from here; not if you want a really nice flying model.
The problem is that you are putting mechanical design priority above aerodynamic design, and making something fit is totally different from making something fly (well).

Highflight

[Kyle G.]

Highflight, after building the wing, I'm beggining to agree, I'm up for more suggestions. so your saying the wing is too thin? if its thicker it will be a little better? I don't want to change the other demensions maybe a 1/2 wider at the chord or a little larger WS. like 44 inches instead of 40 and a 5 1/2 in chord instead of 5? thickness between .75-1 inch? these numbers seem a little better?

[davidfee]

What is the reason for using a symmetrical airfoil?

-David

[Kyle G.]

Alright guys, my previous weight estimations where a little on the high side. Now I'm looking at an AUW of 11-12 oz. Considering the plane empy weighs about 7 oz without floats. And I'm going to use a CDR motor and small lipo. With this reduced weight make it fly a little better? I'm looking for a semetrical aerofoil for looks.

[davidfee]

Lighter weight will nearly always result in better flying qualities. This is especially true for relatively small models.

A semi-symmetrical airfoil would allow you to fly more slowly.

-David

[BMatthews]

I had missed the post about it being a Cessna Caravan style of model let alone the float issue.

Given this new info I seriously doubt you'll be spending much time inverted and that's what the symetrical airfoil would do for you. For a model that will be upright or otherwise only doing positive G maneuvers go for a cambered airfoil for the best performance. Once in the air you'll thank yourself. At that size the basic ClarkY is about what you want.